Nitrogen-based atmospheres have been routinely used by the heat treating industry both in batch and continuous furnaces since the mid 1970s. Because of the low dew point and virtual absence of oxygen, nitrogen-based atmospheres do not exhibit oxidizing and decarburizing properties and are therefore suitable for a variety of heat treating operations. More specifically, a mixture of nitrogen and hydrogen has been extensively used for annealing of low to high carbon and alloy steels, annealing of ferrous and non-ferrous metals and alloys such as carbon steel, copper, copper alloys, gold alloys, and sintering metal and ceramic powders. A mixture of nitrogen and a hydrocarbon gas such as methane and propane has gained wide acceptance for neutral hardening and decarburized-free annealing of medium to high carbon steels. A mixture of nitrogen and methanol has been developed and used for carburizing low to medium carbon steels. Finally, a mixture of nitrogen, hydrogen, and/or moisture has been used for brazing metals and sealing glass to metals.
A portion of nitrogen used by the heat treating industry is produced by distillation of air in large cryogenic plants.
Likewise, a portion of hydrogen used by the heat treating industry is produced by either partial oxidation or steam reforming of natural gas. Both nitrogen and hydrogen produced by these techniques are generally expensive. Additionally, the nitrogen-hydrogen atmospheres prepared by blending these gases are also expensive. To reduce the overall cost, a large number of heat treaters have been producing nitrogen-hydrogen atmospheres by decomposing (or cracking) ammonia in ammonia dissociators.
Ammonia dissociators generally decompose ammonia into a mixture of nitrogen and hydrogen over a bed of nickel, iron, or nickel/iron catalyst supported on a ceramic material. U.S. Pat. Nos. 3,598,538, 3,379,507, and 4,179,407 describe ammonia dissociators in detail. The catalyst promotes the following ammonia dissociation reaction: EQU 2NH.sub.3 =N.sub.2 +3H.sub.2
This reaction is endothermic and requires heating of the catalyst bed to a temperature ranging from 600.degree. C. to 950.degree. C. from an outside source. The operating pressure of the process generally ranges from 2 psig to 10 psig with a space velocity used for the dissociation reaction generally varying from 500 to 5,000 Nm.sup.3 /h product gas per m.sup.3 of the catalyst. The product gas generally contains a mixture of 25%, nitrogen and 75% hydrogen with some ppm of residual ammonia. The content of unconverted ammonia in the product gas can vary from 30 ppm to 500 ppm depending on the operating temperature, pressure, and space velocity. Furthermore, the amount of unconverted ammonia in the product gas is generally known to increase with an increase in the operating pressure. Therefore, heat treaters generally prefer to operate ammonia dissociators at low pressures (below about 8 psig) to minimize the amount of unconverted ammonia in the product gas.
The concentration of hydrogen in nitrogen-hydrogen atmospheres required for the majority of heat treating operations generally varies from about 0.2 to about 25%. Since cryogenically produced nitrogen is cheaper than nitrogen-hydrogen atmosphere produced by dissociating ammonia, heat treaters normally blend cryogenically produced nitrogen with dissociated ammonia product gas to reduce overall atmosphere cost and to produce nitrogen-hydrogen atmosphere with the desired composition. However, these heat treaters are still experiencing the dilemma of high nitrogen-hydrogen atmosphere cost, thus, it is becoming increasingly difficult for them to compete effectively in world markets.
Since the concentration of nitrogen in nitrogen-hydrogen atmospheres varies from about 75%, to 99.8%, it is conceivable to reduce the overall cost of nitrogen-hydrogen atmospheres by replacing cryogenically produced nitrogen with low-cost nitrogen produced by non-cryogenic air separation techniques such as pressure swing adsorption and selective permeation (membrane). The non-cryogenically produced nitrogen costs less to produce, however, it contains from 0.05 to 5.0%, residual oxygen, making a direct substitution of cryogenically produced nitrogen with non-cryogenically produced nitrogen difficult, if not impossible, for certain applications.
Furnace atmospheres suitable for heat treating applications have been generated from non-cryogenically produced nitrogen by removing residual oxygen or converting it to an acceptable form in external catalytic units prior to feeding the atmospheres into the furnace. Such atmosphere generation methods have been described in detail in French publication numbers 2,639,249 and 2,639,251 dated Nov. 24, 1988 and Australian patent application numbers AU45561/89 and AU45562/89 dated Nov. 24, 1988. These methods require use of external units packed with precious metal catalysts such as palladium and platinum supported on ceramic balls or pellets. These external catalytic units can in principle be used to convert residual oxygen present in non-cryogenically produced nitrogen with dissociated ammonia to moisture and produce nitrogen-hydrogen atmospheres suitable for heat treatment. These reactors, however, result in considerable pressure drop, making it difficult, if not impossible, to mix low-pressure dissociated ammonia stream with non-cryogenically produced nitrogen and flow the mixture through them. Therefore, heat treaters have not considered using dissociated ammonia for deoxygenating non-cryogenically produced nitrogen and producing nitrogen-hydrogen atmospheres suitable for heat treatment.
Based upon the above discussion, it is clear that there is a need to switch from cryogenically produced nitrogen to non-cryogenically produced nitrogen for reducing the overall cost of nitrogen-hydrogen atmospheres for heat treaters that are generating nitrogen-hydrogen atmospheres using ammonia dissociators.